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Convergence of Complex Cognitive Abilities in Cetaceans and Primates

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Convergence of Complex Cognitive Abilities in Cetaceans and Primates WellBeing International WBI Studies Repository 2002 Convergence of Complex Cognitive Abilities in Cetaceans and Primates Lori Marino Emory University Follow this and additional works at: https://www.wellbeingintlstudiesrepository.org/acwp_asie Part of the Animal Studies Commons, Comparative Psychology Commons, and the Other Animal Sciences Commons Recommended Citation Marino, L. (2002). Convergence of complex cognitive abilities in cetaceans and primates. Brain Behavior and Evolution, 59(1-2), 21-32. This material is brought to you for free and open access by WellBeing International. It has been accepted for inclusion by an authorized administrator of the WBI Studies Repository. For more information, please contact [email protected]. Convergence of Complex Cognitive Abilities in Cetaceans and Primates Lori Marino Emory University KEYWORDS Convergence, intelligence, cetacean, primate, mammal ABSTRACT What examples of convergence in higher-level complex cognitive characteristics exist in the animal kingdom? In this paper I will provide evidence that convergent intelligence has occurred in two distantly related mammalian taxa. One of these is the order Cetacea (dolphins, whales and porpoises) and the other is our own order Primates, and in particular the suborder anthropoid primates (monkeys, apes, and humans). Despite a deep evolutionary divergence, adaptation to physically dissimilar environments, and very different neuroanatomical organization, some primates and cetaceans show striking convergence in social behavior, artificial ‘language’ comprehension, and self-recognition ability. Taken together, these findings have important implications for understanding the generality and specificity of those processes that underlie cognition in different species and the nature of the evolution of intelligence. Introduction In his popular 1983 article ‘The Wisdom of Casey Stengel’ Stephen Jay Gould posed the following questions: If we could start the earth’s tape anew, would intelligent creatures again evolve? If other worlds share our basic chemistry and conditions, has intelligence arisen on them [Gould, 1983]? These inquiries embody the question of whether attributes that we would identify as intelligence on a par with humans can arise independently of human evolution. At the core of this question is the phenomenon of convergent intelligence. Convergence is a process by which similarity between species occurs because of adaptation to similar environmental pressures and, by definition, not because of phylogenetic relatedness (homology). Convergence is one kind of homoplasy. The others are parallelism and reversal. Convergent traits are defined as those that evolved independently (with the implicit acknowledgement that, as far as we know, no two groups on Earth are completely independent because all life evolved from a single ancestor). Convergence is also often used as a synonym for analogy, i.e. a similar character in two species that was not present in their common ancestor [Ridley, 1993]. The greater the phylogenetic separation of the two groups the stronger the case for convergence. Convergent evolution is a process that takes place on many levels. The convergent evolution of flight is a superb example of convergence of function without convergence of structure. Bats, insects and birds, as well as the extinct flying reptiles all evolved the capacity for active flight. Yet none of these groups are closely related. All have long separate evolutionary histories and use very different mechanisms for achieving the same behavioral outcome – powered flight. The wing of a bird is not very similar to the wing of an insect or a bat, for instance. Convergence can also occur when two species not only independently evolve the same function, but also the same underlying structural or mechanistic basis for that function. This level of convergence is exemplified by recent findings that two very distantly related groups of marine mammals, whales and seals, have developed the same form of short wavelength color blindness through the same route, that is, specific loss of S-opsin cones in the retina [Peichl et al., 2001]. This particular form of colorblindness, the authors postulate, might be adaptive to coastal species of marine mammals. The two previous examples of convergence reveal that convergence can occur on a functional level without complete structural convergence (active flight accomplished by different ‘wing mechanisms’). Convergence can also occur in structure as well as function, as when color blindness is underwritten by the same physiological mechanism. The fact that convergence may occur at both or just one of these levels is particularly relevant to the forthcoming discussion about convergence of intelligence. There are abundant examples in nature of both structural and functional convergence in morphology, physiology, and simple behaviors. But what examples of convergence in higher-level complex cognitive characteristics, vis-a-vis, Gould’s convergent intelligence, exist? In this paper I will provide evidence that convergent intelligence has occurred in two distantly related mammalian taxa. One of these is the order Cetacea (dolphins, whales and porpoises) and the other is the order Primates and, in particular, the suborder anthropoid primates (monkeys, apes, and humans). The focus on this comparison does not imply that convergence in complex cognitive characteristics has occurred only in cetaceans and primates. However, given our own species’ status as a primate and given the emphasis on ‘intelligence on a par with humans’ in questions such as the ones posed by Gould, I argue that primate-cetacean comparisons are particularly relevant to the question of convergence in intelligence. In this paper I will (1) discuss how cetaceans and primates meet the criteria for convergence, (2) present examples of convergence in complex behaviors and cognitive abilities in these two groups, and (3) discuss the implications of this evidence for the evolution of intelligence in general. Separate Evolutionary Histories and Divergent Brain Organization One of the defining criteria of convergence is that it occurs in groups that are not closely related to each other. Therefore, phylogenetic proximity must be essentially ruled out (despite its continuous nature) for a trait to be considered convergent across two or more groups. The case of primates and cetaceans represents one of the deepest phylogenetic divergences among the mammals. There are two living suborders of Primates: Prosimii (prosimians) and Anthropoidea (monkeys, apes, and hominids). The Anthropoids are the focal primate group in this paper. The earliest primates emerged in the fossil record from an insectivore-like mammal during the late Cretaceous Period between 65 and 70 million years ago (mya). These early small shrew-like animals, identified collectively as Purgatorius, were followed by a highly successful group of archaic primate-like mammals that flourished in the Paleocene and early Eocene epochs, i.e., between 50 and 65 mya. Prosimians, the first undisputed primates, emerged during the early Eocene epoch. Monkeys evolved from a group of prosimians in the very early Oligocene epoch about 37–40 mya. The first primitive apes from which the hominoids (lesser apes, great apes and hominids) evolved appear in the fossil record from around 20–25 mya in the early Miocene epoch. The evolutionary divergence among all of the existing great apes and humans occurred in the last 15 million years. The chimpanzee-hominid split occurred approximately 6 mya and the divergence between bonobos (pygmy chimpanzees) and common chimpanzees occurred as recently as 2.5 mya [Napier and Napier, 1985; Fleagle, 1988]. Cetacea include the two living suborders Odontoceti (dolphins, toothed whales, and porpoises) and Mysticeti (baleen and rorqual whales) and one extinct archaic suborder from the Eocene referred to as Archaeoceti. Cetaceans originated when a hoofed Paleocene land mammal adopted an aquatic lifestyle between 55–60 mya (late Paleocene-early Eocene) and gave rise to the archaeocetes. Archaeocetes radiated into several forms in the Eocene epoch, but were extinct as a group by the beginning of the Oligocene epoch, 37 mya. The earliest odontocetes and mysticetes first appeared in the fossil record at about the same time that the last archaeocetes disappeared. Early modern cetacean species appeared in the fossil record by the mid Miocene epoch and most modern species were established by the onset of the Pleistocene, i.e., 5 mya ago [Barnes et al., 1985; Fordyce and Barnes, 1994; Bajpai and Gingerich, 1998; Thewissen, 1998]. Fig. 1. Illustration of the phylogenetic relationships among several major living primate and cetacean groups and their estimated time of divergence. Mya = million years ago. It is clear from the fossil record that the divergence between cetaceans and primates had already occurred at least 65 mya. However, the divergence between the ancestral mammal groups that eventually led to cetaceans and to primates probably occurred even earlier in the Mesozoic Era 90–95 mya [Cooper et al., 1990; Bromham et al., 1999] although the mammalian fossil record becomes increasingly sparse the further back in time. Figure 1 shows the phylogenetic relationships among major living primate and cetacean groups and the estimated time of their divergence. During their long independent evolutionary histories primates and cetaceans adapted to radically different physical requirements (arboreal/terrestrial
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